Speed to fly
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Speed to fly is a principle used by soaring pilots when flying between sources of lift, usually thermals, ridge lift and wave. The aim is to maximize the average cross-country speed by optimizing the airspeed in both rising and sinking air. The optimal airspeed is independent of the wind speed, because the fastest average speed achievable through the airmass corresponds to the fastest achievable average groundspeed.
MacCready speed to fly ring for a variometer. The outer ring show various airspeeds, while the variometer shows climb rate. The index arrow, white triangle, on the ring is placed against the expected rate of climb at the next thermal. The variometer needle will then point to the optimum airspeed, listed on the ring, to be flown to that thermal. The greater the expected rate of climb, the more clockwise the ring is rotated, and the faster is the optimum airspeed.
Drag curve
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The drag curve or drag polar is the relationship between the drag on an aircraft and other variables, such as lift, the coefficient of lift, angle-of-attack or speed. It may be described by an equation or displayed as a graph. Drag may be expressed as actual drag or the coefficient of drag.
Coefficients of lift and drag against angle of attack.
Polar curve for a glider, showing glide angle for minimum sink rate. The origin of the graph is where the airspeed axis crosses the sink rate axis at zero airspeed and zero sink rate. The horizontal line is tangent to the top of the polar curve. That tangent point indicates the minimum sink airspeed (vertical line). The sink rate increases to the left or right of this point, corresponding to a lower or higher airspeed. This minimum sink airspeed has the lowest possible rate of sink, and allows the longest possible glide time before landing.
Polar curve for a glider, showing glide angle for the best glide speed (best L/D). It is the flattest possible glide angle through calm air, which will maximize the distance flown. This airspeed (vertical line) corresponds to the tangent point of a line starting from the origin of the graph. A glider flying faster or slower than this airspeed will cover less distance before landing.